In some applications, such as medical imaging, a low power color sequential camera head, using a monochrome sensor, provides a digital output signal over a low bit rate digital link to a remote base unit. A typical application is an endoscope for examining body cavities, wherein the camera head is at the end of a probe, and the base unit includes a monitor for viewing images captured by the camera head. The camera head sequentially illuminates the scene with red, green, and blue light, and then sequentially digitizes the red, green, and blue color images output from the monochrome sensor. In addition, to minimize power, the pixel rate of the digital output signal should be as low as possible, while still maintaining good image resolution and good motion portrayal. Ordinarily, the RGB images all use the full sensor resolution. The monochrome sensor is read out in an interlaced mode at 60 fields per second, so that there are 20 red, 20 green, and 20 blue fields per second.
A known type of field sequential color television system, for use in a known type of endoscope, is shown in U.S. Pat. No. 4,845,553. The light of three primary colors (red, green, and blue) is sequentially irradiated onto an object such as an organism, and the reflected light of each of the individual colors is received by a charge-coupled device (CCD), where it is converted into an electrical image signal. After the thus-obtained picture signals are stored in memories in succession, they are converted into color television signals by a processor and displayed on a color monitor. In particular, as shown in the '553 patent, the memory capacity is reduced by sub-sampling the full resolution output of the CCD for the red and blue light, while maintaining the green light at its full resolution output.
In U.S. Pat. No. 4,685,451, red and blue light is similarly sub-sampled, but within the same timing. This is done by utilizing a single sensor having cyan and yellow color filters. Full resolution green is obtained for odd fields by illuminating the sensor with green light (cyan and yellow photosites both being sensitive to green light), and red and blue are obtained at lesser resolution for even fields by illuminating the sensor with white (or magenta) light (cyan and yellow photosites being separately sensitive to blue and red light, respectively). The light may be input into the endoscope sheath by means of fibers, or, as described in U.S. Pat. No. 4,074,306, a package enclosing a plurality of solid state light emitting chips can be mounted in the sheath of the endoscope.
In the known systems as described, including ones that use sub-sampling to reduce resolution and storage, the pixel data rate nonetheless must be maintained at relatively high rates, such as 12 MHz or thereabouts. Moreover, subsampling can reduce image quality by introducing color aliasing artifacts. To provide an improved camera, what is needed is a lower pixel rate, to decrease power consumption, yet a higher field rate, to improve motion rendition--without producing color artifacts.